Rain Harvest is a simple calculator designed to estimate potential rainwater runoff. Values for catchment area, rainfall, efficiency and number of collection points are entered into the calculator and results are displayed in either US or Metric units. Individual calculations can be saved, edited, deleted and searched.

Any measurable surface that rain falls on such as a roof, patio, etc.
To calculate a rectangular catchment area, simply multiply the length times the width.

Rainfall

The amount of rainfall expressed in either inches or millimeters.

Efficiency

Rain runoff efficiency expressed as a percentage. Runoff efficiency is influenced by many factors including the composition of the surface it is falling on, air temperature, evaporation rate, etc. If unsure, a good estimate is 75%.

Collection Point

Collection points are the locations where rainwater runoff is collected after it flows off the catchment area.
Collection Points might consist of rain barrels, cisterns, tanks, or anything else that collects water for later use.

Displays more detailed information about your calculation. The "Rule of Thumb" shows how the calculation is made in general terms with examples of the formula below. Finally, "Your Calculation" displays the calculation within the context of the actual mathematical formula used to perform the calculation.

You can email and print your calculations by tapping the Action icon located in the toolbar. You can send your calculations to anyone via email or print your calculation from the device.

Please Note: Sending calculations via email and printing from the device requires an active WiFi or cellular connection. Printing calculations from the device also requires an AirPrint compatible printer or a printer sharing utility like Printopia.

For example, you might typically perform calculations using 1 collection point with a 75% efficiency rate. You can save your preferences so that each time you create a new calculation these fields are pre-populated with the defaults.

Title - enter a short title.

Catchment Area - enter a valid whole or decimal number.

Rainfall - enter a valid whole or decimal number.

Efficiency - enter a valid number that represents a percentage from 0.01 to 100. Whole or decimal numbers are allowed.
For example, entering the number 75 means 75%.

Collection Points - enter a valid whole number. Number must be 1 or greater.

US / Metric Unit Selector Button - select US or Metric.

Include notes in detail - setting this to On will include the contents of the Note field when printing or emailing the calculation.

Rainwater harvesting is an old-fashioned and simple way of collecting rainwater. For years people have channeled rainwater into cisterns and water barrels. By connecting the downspouts from roof gutters to some form of drum-like water barrel, people can quickly and easily collect rainwater for later use. This same basic set-up, with a few adaptations, is useful in city gardens where getting water can sometimes be difficult.
Only a few simple items are required: rain barrels, gutters, tubing, hardware to connect barrels, and materials to label barrels. Exactly how many or much of each is needed will depend on your design. Each water harvesting system will vary with the given garden's access to roof run-off, the amount of space available for the system, and the amount of water you hope to collect.

As you think about designing a water harvesting system, there are a few basic questions you should answer or at least keep in mind:

Will you collect water from a roofed structure in your garden, or from the roof of an adjacent building?

A system that collects water from the roof of a casita or tool shed is the simplest. They usually do not have as much surface area, and so will not yield as much water as a building roof. A system that runs off a building's down-spout will usually yield more water, but will also require that you get permission from the building owner and, if you are going to fill a number of barrels, will require that you have a strategy for managing the overflow.

How much water will you harvest?
You will be surprised how much water you will be able to collect from event the smallest roof. The rule of thumb is 623 gallons of water per inch of rain per thousand square feet of catchment area. You will need to calculate the footprint of the building. In other words if your building is 25 feet by 50 feet the catchment area is 1,250 square feet (25 sq. ft. x 50 sq. ft.) Not all the rain that falls can actually be collected. Efficiency is usually presumed to be 75% depending on system design and capacity. Here is the basic formula for calculating the potential amount that can be collected:

To lookup average precipitation in your area visit the National Oceanic and Atmospheric Administration at http://www.noaa.gov/. Keep in mind that during a drought there will be less rainfall.

How many barrels do you want to fill?
There is no limit to the number of barrels you can put in your system. Simple tubing can connect them all.

Will the barrels be used for dipping into with a bucket of some sort, or will the barrel have a spigot and/or hose attachment?
It is great to be able to hook to your water barrels for watering. Keep in mind, though, that you will be relying on gravity or waters tendency to always seek the lowest point to produce water pressure, so the nozzle of the hose will always have to be lower than where the hose connects to the barrel. It is good to have the barrels higher up. You can do this in a few simple ways:

Simply situate the barrels at the highest point in your garden, if your garden has any slope to it.

Build a berm and a retaining wall to set the barrels on.

Build a frame to stack the barrels on.

Remember that even a water barrel that is elevated on some sort of platform wont give you pressure comparable to what you get from the hydrant.

What will you do with the overflow?
Chances are that no matter how many barrels you put in your system, there will sometimes be rain that is heavy enough to overflow the last barrel in the bunch. There are various ways of handling the overflow and integrating it into your garden plan:

Put the barrel that will overflow in a bed of sand and gravel. These ground surfaces will absorb excess water.

Build a mini streambed, lining it with rocks, to channel overflow to a low spot in the garden. The best way to figure out where to build a small streambed is to let the barrel overflow and watch where the water naturally flows. You will need to make sure that the water will be absorbed and not create a standing water situation.

Design your system so that the overflow from the last barrel automatically goes back into a downspout and into the sewer pipe where the buildings roof water drains.

What are the Department of Health recommendations affecting rainwater collection, in regards to the West Nile Virus?
For instance, the following are some basic recommendations from the Department of Health's West Nile Virus Fact Sheet:
Dispose of tin cans, plastic containers, ceramic pots, or similar water-holding containers.
(Gardens may have water-holding containers but they must be covered with tightly fitting lids, with no standing water exposed.)
Make sure roof gutters drain properly. Clean clogged gutters in the spring and fall.
Eliminate any standing water that collects on your property.

When building your rainwater collection system you will need to take the following precautions:

Use rain barrels with tight fitting lids and keep them closed at all times.

Use tight fitting, fine mesh screens to cover small ventilation holes on tops of barrels.

Cover the overflow pipe with a piece of fine mesh. Do not allow water to pool up in overflow area.